Transmission frequency-delay-measuring circuit



Dec. 9, 1930. D. MITCHELL.

TRANSMISSION FREQUENCY DELAY MEASURING CIRCUIT Filed Oct. l2, 1928'Patented Dec. 9, 1930 UNITED sTATEs PATENT OFFICE l l DOREN MITCHELL, FNEW YORK, N. Y., ASSIGNOB T0 AMERICAN `U.ELEPHONE AND TELEGBAPH COMPANY,A. CORPORATION 0F NEW YORK Application led October 12, 1928. Serial No.312,188.

This invention relates to means for measzuring the transmissioncharacteristics of telephone or telegraph systems and the like, and imore specifically discloses means for measur- 6 ing the difference intime re uired for transmltting any two distinct requencies over such atransmission system.

The difference in time required for transmitting different frequenciesover a transmis- 10 sion line has been termed the relative delay. Therelative delay of the various essential frequencies in signaltransmission compared to some reference frequency furnishes one of thecriteria as to the amount of distortion introduced by the circuit duringsuch signal transmission. It will be seen therefore that a method foraccurately measuring the relative delay versus frequency of atransmission system has important applications in connection with thedesign of equalizing networks adapted to reduce the amount of distortionin a given transmission system.

A brief dlscussion will be given here to bring out the relationshipbetween the delay Se@if-frequency characteristic of a transmissionfrsystem and the amount of signal distortion introduced by y suchsystem. It is a Well- .known fact that fora perfectly distortionlessine, all frequencies are attenuated by a like amount and are'transmitted with the same velocity of propagation. If upon such a line,properly terminated in its characteristic impedance, a current of givenfrequency is suddenly im ressed at the sending end, a definite period otime will elapse before any effect is noted at the receiving end andthen the received current will suddenly start right off at its finalsteady state value. Stated somewhat diiferently, a perfectlydistortionless line has the important steady state characteristics thatall frequencies are attenuated equall during transmission over such lineand tie total phase change varies linearly with frequency.

A line containin distortion, on the other hand, differs signi cantlyfrom a distortionless line in that if, u on a line producing distortion,a current of) given frequency is sudperiod of time elapses before anyeffect is denly impressed at the sending end, a certainl noted at thereceiving end and then v4[the received current builds up gradually toits final steady state value. This gradual building up of the receivedcurrent is the direct result ofk the distortion introduced by thecircuit and is determinable from the steady state characteristic of theline, i. e., the steady state frequency-attenuation curve and the mannerin which the total phase shift varies with fre.- quency. If the line .beequalized as regards steady state attenuation,that is, if suitablenetwoiks are inserted to make the overall attenuation approximatelyconstant with frequency, then the building up time of the receivedcurrent is determined solely by the relationship betweenv total phasechange and frequency. In such case, the departure of the total phasechange-frequency curve from linearity ives a measure of the amount ofphase distortion introduced by the line.` A line equalized as regardssteady state attenuation has the peculiar characteristic that current ofany frequency in the equalized .range applied to the sending end willbuild up to of its final state value at'the receiving end in an intervalof time which is equal to the first derivative of the total phase changefrequency-curve at such frequency forthe circuit under consideration. Itwill thus be seen from this relationship that there is a directconnection between the building up time of the received current and theamount of phase distortion introduced by the circuit. And as a vresultof this relationship, it will be seen further that a measurement of thefrequency delay curve of the circuit furnishes an experiphasedistortion.

The theoretically distortionless line is never encountered in actualpractice. The best that can be' done is to make the line distortionlessover the essential range of signaly frequencies.. For example, in speechtransmission, a line would be practically distortionless in which theattenuation was constant from a fr uenc of zero to about- 10,000 cyclesandefor w ich the total phase change varied linearly with frequency oversuch range. A line of this type equalized over a certain frequency rangemight be det -mental method of determining the amount of fined as acommercially distortionless line.

The commercially distortionless line defined as above has the peculiarcharacteristic' that all frequencies within the equalized range willbuild up to their proximate steady state values in the same interval oftime. The time required for the received current to build up to 50% ofits final steady state value will be defined herein as the proximatesteady state current. In other words, the delay frequency curve for sucha circuit over the equalized range measured to the time currents reachtheir proximate steady state values would be a straight line coincidingwith the frequency axis. Consequently, if a delay frequency curve wereobtained for a line producing appreciable d1stortion in the essentialfrequency range in such a manner that the delay in each caserepresented, the relative time required for each frequency to reach itsproximate steady state value as compared to the time for some referencefrequency, the resultant curve would measure directly the amount ofphase distortion introduced bythe circuit, assuming, of course, such aline to be equalized to have approximately constant attenuation `overthe essential frequency range.

The above statements regarding the transient characteristics oftelephone lines are merely stated as facts without proof. Themathematical derivations of such statements are disclosed in thefollowing papers: (1) Phase distortion and hase distortion correction',by S. P. Mea in the Bell System 4System Technical Journal., vol. 3, No.4, at

page 558.

Having described above the value, in the art of electrical communicationof a knowledge of the frequencydelay characteristics of a transmissionsystem, the means by which it is proposed to measure such delaycharacteristics will now be disclosed; Before taking up such discussion,however, the advantages of the present disclosure over the prior artwill be pointed out. The present invention discloses a method ofdetermining directly the difference in time required for transmittingany two frequencies over a transmission line; whereas in the majority ofmethods now known, the apparatus measures separately the total time oftransmitting each fre uency, the diderence in time being obtaine bysubtraction. The latter method has the serious disadvantage that a smallerror in determining the total time of transmission will become arelatively large error in determining the relative delay, i. c., thedi'erence in time of transmitting two frequencies.v

In the drawings, Figure 1 discloses a circuit arrangement for measuringthe relative delay of a given transmission line by the method of thepresent invention. Fig. 2 discloses an alternative means for applying atthe sending end the frequencies to be compared.

Referring to Fig. 1, sources of frequency {.1 and 2 are associated withtransmission me 5 or which the delay is to be '.etermined. The closureof key 9 completes a circuit to operate relay 8, the operation of whichsimultaneously applies fre uencies f1 and i, to the test circuit 5throng filters 1 and 2. These filters are band-pass filters, filter 1being designed to pass a small range of frequencies including f1 andfilter 2 similarly designed vto transmit a small range includ. ing f2..These filters are inserted to eliminate some of the transientfrequencies which are introduced by the sudden building up of currentsf, and f2 in the circuit.

The frequencies f1 and f2 thus simultane ously applied are transmittedover the' test circuit 5 from the sending end AA to the l distant endBB, at which point the frequencies are separated by means of filters 3and 4. Filter 4 is a band-pass filter which passes a .small range offrequencies including f2. Filter 3 is a band elimination filter whichellminates a small range of frequencies including ,f2 but passes allothers. Thus, upon arrival of the current at thereceiving end, frequencyF1 ows through iilter 3 and is amplified and rectified in device 6.Frequency f2 flows through ilt'er 4 and is amplified and rectified indevice 7. With switch 16 suitably operated, as will be explained below,the rectified currents of the two frequencies operate relays lO'and 11,respectively, thus suitably operating the delay measuring circuit.

At this point, it seems advisable to explain 'the manner in which ,thetiming circuit measures the delay. The timing circuit comprisesapparatus 19 together with relays 10 and 11. This timing circuit formeasuring short intervals of time is described in a paper by J. Herman,entitled Bridge for measuring small time intervals, in the Bell SystemTechnical Journal for April, 1928, vol. 7, No. 2, at page 343. Theollowingexplana tion is given there of the manner in which the devicefunctions. Two condensers C1 and C2 are charged from the common batte12. The condenser C1 is charged throng-liI an adjustable high resistanceR1 during the time elapsing between the operation of relay 11 and thesubsequent operation of relay 10. This elapsed time is the interval oftime to be measured and the charge accumulated on the condenser C1during such time is an accurate means of measuring the time. The secondcondenser C2 is used merely for oomparison purposes. It is chargedthrough a fairly low resistance R, and acquires its full charge in arelatively small interval of time.

After the completion of the charging interval, relay 18 is operated andthe two condensers are discharged simultaneously through thedifferential meter circuit comprising meter 17 and similar resistances14 and 15 connected as shown. If the charges on the two condensers areequal, the meter 17 will show no deflection but if they are unequal themeter will show a momentary deflection, the direction of which willindicate whether the charge on C1 is too high or too low. By repeatingthe charge and discharge process a few times, meantime adjusting thevalve of resistance R1 in series with condenser C1, the charge' on thetwo condensers can be made equal and the meter will thus show nodeflection. When this condition is obtained, the interval of time duringwhich the charging took place ma be determined from the value of the higresistance R1.

The relationship between the interval of time of charging and the valueof the re sistance R1 required to make the charges on the two condenserse ual is a direct proportion. This will be o vious from the inspectionof the equation for the char e at any instant on a condenser which ising charged through a high resistance. This equation is:

where g'equals the char e at'time t, t equals the lapsedtime in secondssince the charging began, Q equals final or maximum charge on thecondenser, C1 equals capacity of the condenser and R1 equals the seriesresistance.

Since g, by adjustment of resistance R1 is always made equal to thecharge on the comparison condenser and sincethe charging attery iscommon to the two condensers, therefore using the symbols shown on thedra-Wing for circuit 19 02E may be substituted for g and C1B for a Q,the equation then becomes:

As mentioned, the two condenser capacities are kept constant..Therefore, any change in t requires a proportional change in Rl 1norder to satisfy the equation.

Returning now to the original discussion, assume that switch 16 isthrown to the left and that frequency f2 builds up to its proximatesteady state value at the receiving end before frequency f1. Theamplifier-detectors 6 and 7 are so adjusted that relays 10 and 11 willbe-operated by the rectified proximate steady state currents offrequencies f1 and f2, respectively. Frequency f2 under the aboveassumption operates rela 11 first. This starts the charging of con enserCl over the battery 13 throughthe winding of relay 18,

front contact of relay 10 and front contact of relay 11, which is still`operated, to round. The operation of relay 18 connects t e condensers C1and C2 in the differential meter circuit as explained above, thuspermitting the discharge thereof, and the defiection of the needle ofmeter 17 indicates Whether resistance R1 is adjusted to 'a proper valueor not. By opening and closing key 9 at intervals, thus transmittingfairly long pulses of frequencies f1 and f2, meantime adjustingresistance R1, a balance can be obtained so that the meter 17 shows nodeflection. From the setting of resistance R1, when the balance isobtained, the delay of frequenc f1 as compared to f2 is determineddirect y in seconds through proper calibration of theA re 16 thrown tothe left, frequency ,f1 arrived beve' fore frequency f2, relay 10 Wouldfirst be operated, followed by relay 11. Under this condition, no chargewould ever be accumulated on condenser and hence no balance could beobtained. In such case, it 'would be necessary to throw switch 16 to theright, thus reversing the relays by connecting relay 10 to detector 7and relay 11 to detector 6. With this arrangement, frequency f1 `arrivinfirst would operate relay 11 and subsequent y the arrival of frequencyf2 would operate relay 10, in which case a balance on meter 17 could beobtained and the delay thus determined.

Since there is delay in the filters and in the measuring circuit as awhole, it is necessary that the delay measuring circuit be calibrated.for zero test circuit, i. e., with the circuit at AA connected directlyto the circuit at BB. With this arrangement and the frequencies f1, f2applied, the delay of the measuring circuit is determined once for alland a suitable correction equal t0 this delay is thereafter applied toall measurements made with a test circuit 5 connected.

Frequencies f1 and f2 may, of course, be any two frequencies with theresult that filters 1 to 4, inclusive, must be adjustable or the samefrequency f2 may be used in all cases as a. reference frequency andfrequency f, only varied, in which case filters 2, 3 and 4 could befixed and filter 1 variable.

If frequencies f1 and f, are well separated,

- key 21 closes t e circuit to operate-relay 22 m which in turn, closesthe circuit to simultaneousl apply frequencies f1 and f2 to line 5through transformer 23. The arrangement of Fig. 2 is preferable to thatof Fig. 1, due to the possibility of error being introduced in 1 thelatter case through failure of the simultaneous closure of the contactsof relays 8.

With the arrangement of Fig. 2, it would robablybe necessary to use tworeference requencies, say of 1,000 and 2,000 cycles,

otherwise when the comparison and reference frequencieswere closetogether, the transients due to one frequency might operate prematurelythe receiving apparatus associated with the other frequency. To avoidthis, fre- 25 uency f2 might be adjusted at 1,000 cycles or ymeasuringthe delay of frequencies above 1,500 cycles, and adjusted at 2,000cycles for measurin the delay of frequencies below 1,500 cyc es. i

Whatis claimed is 1. The method of measuring the relative delay of atransmission circuit, which consists in simultaneously impressing uponthe sending end of said circuit a pair of distinct single frequenccurrents, separating said frequencies at t e receiving end to facilitatemeasurement thereon, and measuring the difference in time of arrival ofsaid frequencies.`

. 2.1The method of measuring the relative delay of a transmissioncircuit, which consists in simultaneously impressing upon circuitsindividual thereto, a air of distinct single frequency currents, ltering such currents to suppress the extraneous frequencies, impressingsaid filtered currents upon the sending end of the transmission circuit,separatin said frequencies at the receiving end to faci itatemeasurement thereon, and measuring the difference intimeof arrival ofsaid currents.

3. The method of measuring the relative delay of a transmissioncircuit,which consists in simultaneously impressing upon the sending end of saidcircuit,a pair of distinct sys r single quencies 'at the receivin end tofacilitate measurement thereon, an measuringthe difference in time-required for said received frequencies to build upto apredeterminedfraction of their finalsteady state values.

4. The method of measuring the relative delay of a transmission circuitwhich consists in simultaneously impressing upon the sending end of saidcircuit, a pair of distinct sinequency currents separating said fre- 66'gle frequency currents separating said fresists in simultaneouslyimpressing upon circuits individual thereto a pair of distinct singlefrequency currents, filtering such currents to suppress the extraneousfrequencies, impressing said filtered currents upon the sending end ofthe transmission circuit, separating said frequencies at the receivingend to facilitate measurement thereon, individually amplifying andrectifying said frequencies thereat, causing each said rectified currentto operate individual rela means upon building up to a predetermineportion of its final steady state value, and operating time indicatingmeans in accordance with such relay operations to measure theintervening time interval.

6. Means for measuring the relative delay of a transmission circuit,comprising in combination a plurality of sources of distinct Singlefrequency currents, means for simultaneously impressing said frequenciesupon the transmission circuit, means at the receiving end for separatingthe frequencies to facilitate measurement thereon, and timing means formeasuring the differences in times of arrival of said currents.

7. Means for measuring the relative delay of a transmission circuit,comprising a current source of reference frequency, a second currentsource adjustable in frequency, means for simultaneously impressing onthe transmission circuit at the sending end current from both saidsources, means at the receiv# ing end for separating the transmittedfrequencies, and timing means thereat for meas- 'uring the difference intime of arrival of said frequencies.

8. Means for measuring the relative delay of a transmission circuit,comprising in combination a pair of distinct single frequency currentsources, means for simultaneously impressing said frequencies upon thesending end of said transmission circuit, filter means at the receivingend for passing the received frequencies into channels individualthereto, amplifying rectifying means individual to each said channel,relay means individual to leach rectifier output operable upon saidrectified current attaining a predetermined fraction of its final steadystate value, and timing means operated in accordance with the successiveoperation of said relay means for measuring the intervening timeinterval. 9. Means for measuring the relative delay of a transmissioncircuit, comprising in combination a pair of distinct single frequencycurrent sources, means for simultaneously impressing said currents onthe transmission circuit, means at the receiving end for passing eachfrequenc into a channel individual thereto, ampli er rectifier meansindividual lo to each channel, relay means individual to each rectifieroutput operable upon said rectifier current attaining a predeterminedfraction of its final steady state flow, timing means operated inaccordance with the successive operations of said relay means formeasuring the intervening time interval, said timing means comprising asource of potential for charging a first capacity to the potentialthereof and for charging a second capacity through an adjustableresistance during the interval between the successive relay ope-rations,Ameans thereafter to simultaneously discharge said capacities through adifferential meter circuit, means to adjust said variable resistanceuntil said meter shows no deflection upon discharge of said capacities,and a calibration for said adjustable resistance to indicate the delay.

10. Means for measuring the relative delay of a transmission circuit,comprising in combination a pair of distinct single frequency currentsources, filtering means individual to each current source foreliminating extraneous frequencies, a connection from the output of eachfilter to the sending end of the transmission circuit, means forsimultaneously impressing said currents upon said filters fortransmission over the circuit, filter means at the receiving end forpassing the received frequencies into channels individual thereto,amplifying rectifying means individual to each said channel, relay meansindividual to each rectifier output operable upon said rectified currentattaining a predetermined fraction of its final steady state value,timing means controlled in accordance with the successive operation ofsaid relay means for measuring t e intervening time interval, areversing switch interposed between said rectifying means and said relaymeans for connecting the relays to the rectier means in proper sequenceto insure proper operation of said timing means for measuring the delay.In testimony whereof, I have signed m name to this specification this9th day of October, 1928.

DOREN MITCHELL.

